Historically, local area computer networks (LANs) using optical data links have relied on light emitting diode (LED) sources launching into multimode optical fibers. The EIA/TIA and IEC Building Wiring Standards (TIA 568A) specify the use of 62.5/125 micron multimode optical fiber for intra-building wiring. These standards have resulted in the large-scale deployment of multimode optical fiber in existing computer networks.
In prior communication application technologies, these data transmission platforms have provided adequate bandwidth. Asynchronous transfer mode (ATM) computer networks can support data transmission rates as high as 622 megabits/sec (MBPS), but LED rise times, the chromatic dispersion associated with the relatively wide bandwidth of light produced by the LEDs, and multiple fiber transmission modes impose an upper cap on the potential data rates. Thus, LED/multimode fiber systems are generally limited to sub-gigabit/second (GBPS) data rates.
Newer computer applications requiring higher bandwidths and the increasing number of users that must be serviced by individual networks have led the push to provide GBPS performance, and better. In order to attain this performance in the context of existing optical data links, the LED light sources have been replaced with single mode sources such as vertical cavity surface emitting lasers (VCSEL) and Fabry-Perot lasers. These devices can produce the necessary rise times and have the narrow spectral widths required for GBPS data transmission speeds.
Computer network links modified to use single mode laser sources, however, many times still fail to achieve the data/error rates at GBPS data rates that would be predicted solely from the laser source performance. The problem has been traced to computer links using multimode optical fiber. In many instances, a pulse-splitting phenomena is detected, which increases the bit error rates to unacceptably high levels at these speeds.
The obvious solution to this problem is to use single mode fiber with the single mode sources. While being viable for newly installed computer networks, such a solution is impractical for the installed base of multimode fiber networks since running new fibers in and between buildings represents a significant expense.
Other solutions have been proposed to constrain pulse splitting in signals from single mode sources that have been launched into multimode fibers. In one case, the signal from the single mode source is launched into a short-length pigtail of single mode fiber. The other end of this fiber is then coupled to the existing multimode fiber, offset from the multimode fiber core center.
The problem with the offset single mode-multimode fiber coupling solution is the difficulty of implementing it in the typical computer network environment. The single mode fiber must be precisely misaligned to the multimode fiber such that the light is still launched into the multimode fiber with acceptable efficiency, and this misalignment must be maintained in the coupling module across its lifetime.
According to the present invention, pulse splitting is constrained in single mode source/multimode fiber systems by preventing light from the center of the multimode fiber from being transmitted to the detector. When this is achieved, the detector is insulated from the effects of any pulse splitting, supporting data rates of greater than one GBPS by increasing the modal bandwidth.
In general, according to one aspect, the present invention features a method for improving modal bandwidth in an optical link, such as in a computer optical network, using a multimode optical fiber. The method comprises generating an optical signal with a single mode laser source and coupling the optical signal into the multimode optical fiber. The optical signal from a center portion of the optical fiber, however, is blocked from reaching a detector of the optical signal.
In one implementation, the source is a Fabry-Perot or vertical cavity surface emitting laser.
In specific embodiments, an opaque spot is inserted between the laser source and the detector to block the center of the optical fiber from transmitting a detectable optical signal. As such, the spot is applied to a fiber coupler or the fiber of the network. Further, the spot may be applied to either the entrance or exit apertures of the fiber. In any case, the spot should be approximately 4 to 7 microns in diameter.
Alternatively, a fiber coupler with a dark central core is also useful. It can be inserted either at the detector or laser source end of the optical fiber, or both.
According to another aspect, the invention features multimode optical fiber of the computer network with at least one opaque spot for blocking the optical signal from a center portion of the optical fiber from reaching the detector.
Finally, according to another aspect, the invention also features a fiber coupler with a dark core for blocking the optical signal from a center portion of an installed multimode optical fiber from reaching a detector.
The above and other features of the present invention, including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.